1. Field of the Invention
The present invention relates to a surface mount quartz crystal oscillator having a quartz crystal unit with a mounting substrate joined to the reverse surface thereof, and more particularly to a method of manufacturing a mounting substrate and a surface mount crystal oscillator which employs such a mounting substrate.
2. Description of the Related Art
Surface mount crystal oscillators which have a crystal unit and an IC (Integrated Circuit) chip having an oscillating circuit that employs the crystal unit and are shaped for use in surface-mounted applications are small in size and weight, and hence are widely used as a time or frequency reference source in communications devices such as cellular phones or the like. Particularly, a surface mount crystal oscillator constructed as a temperature-compensated crystal oscillator (TCXO) is suitable for use as a time or frequency reference source in portable devices because it generates a stable oscillating frequency in mobile environments that undergo large temperature changes. One known surface mount crystal oscillator is of the joined type wherein an IC chip is mounted on a mounting substrate and joined to the reverse surface of a crystal unit, as disclosed in Japanese laid-open patent application No. 10-98151 (JP-A-10-098151) for example.
FIG. 1A is a cross-sectional view of such a conventional surface mount crystal oscillator, and FIG. 1B is a plan view of a mounting substrate used in the surface mount crystal oscillator shown in FIG. 1A.
As shown in FIGS. 1A and 1B, the conventional surface mount crystal oscillator comprises crystal unit 1 and mounting substrate 2. Crystal unit 1 has substantially rectangular casing 3 of laminated ceramics having a recess defined therein, quartz crystal blank 4 disposed in the recess, and cover 6 placed on casing 3 and hermetically sealing crystal blank 4 in the recess. Crystal blank 4 comprises a substantially rectangular AT-cut quartz crystal blank, for example. Excitation electrodes (not shown) are formed respectively on the principal surfaces of crystal blank 4, and extension electrodes (not shown) extend from the respective excitation electrodes to opposite sides of one end of crystal blank 4. The sides of the end of crystal blank 4 to which the extension electrodes extend are fixed to a pair of connecting electrodes (not shown) formed on the bottom surface of the recess in casing 3 by electrically conductive adhesive 7. Crystal blank 4 is thus held horizontally in casing 3 and electrically and mechanically connected thereto.
Connecting terminals 5 are formed on the four corners of an outer bottom surface, i.e., a reverse surface, of casing 3 and are electrically connected to crystal blank 4. Connecting terminals 5 are electrically connected to the connecting electrodes on the bottom surface of the recess in casing 3 by electrically conductive paths in casing 3.
Mounting substrate 2 has a substantially rectangular planar shape which is essentially identical to or slightly larger than crystal unit 1, with a recess defined in one principal surface thereof. Specifically, mounting substrate 2 comprises bottom wall 8 in the form of a substantially rectangular plate and frame wall 9 in the form of a substantially rectangular frame. Frame wall 9 serves as a side wall of the recess. Mounting substrate 2 is made of ceramics with frame wall 9 having an opening and laminated on bottom wall 8. Connecting terminals 10 are formed on the upper surface of frame wall 9 which surrounds the recess in alignment with respective connecting terminals 5 on casing 3. Connecting terminals 5 on crystal unit 1 and connecting terminals 10 on mounting substrate 2 are electrically and mechanically connected to each other by solder thereby to join the open side of mounting substrate 2 to the bottom surface of crystal unit 1, completing the surface mount crystal oscillator.
IC chip 12 has a plurality of terminals on one principal surface thereof. IC chip 12 is fixed to bottom wall 8 of mounting substrate 2 by ultrasonic thermo-compression bonding with bumps 11, for example, interposed therebetween. The terminals of IC chip 12 are electrically connected to a circuit pattern on bottom wall 8 of mounting substrate 2 by bumps 11. Resin 13 is poured into the recess in mounting substrate 2 to protect IC chip 12, in particular, the surface of IC chip 12 that is joined to bumps 11.
In the crystal oscillator, IC chip 12 comprises a high-functionality IC chip and incorporates therein, in addition to the oscillating circuit, a temperature compensating mechanism for compensating for temperature-dependent changes of an oscillating frequency and a PLL (Phase-Locked Loop) control circuit for producing a signal having a desired frequency from the original oscillating frequency signal.
Mounting terminals 14 including a power supply terminal, a ground terminal, and an output terminal are disposed on the four corners of an outer bottom surface of bottom wall 8. Mounting terminals 14 are used to mount the surface mount crystal oscillator on a wiring board.
As more and more efforts are made to reduce the size of the outer planar shape of the surface mount crystal oscillator, it becomes more and more difficult to place the IC chip 12 in the recess in mounting substrate 2. Particularly, if a high-functionality IC chip incorporating a PLL control circuit is employed, then since its outer profile is greater than the output profile of ordinary IC chips for use in TCXOs, it is difficult to place such a high-functionality IC chip in the recess in mounting substrate 2.
In Japanese patent application No. 2002-228801 (JP, P2002-228801), the assignee of the present invention has proposed mounting substrate 2 having frame wall 9 with one side thereof removed to make the recess open laterally in the open side of frame wall 9, thus virtually increasing the area of the bottom of the recess for accommodating an IC chip therein, as shown in FIG. 2. With the proposed structure, however, when a resin is poured into the recess for the purpose of protecting the IC chip, the poured resin tends to flow out through the open side of frame wall 9. As a result, mounting substrate 2 is likely to suffer a defect such as a poor appearance, and the productivity for such mounting substrates 2 is low.